This is the upgrade they are talking about! French Wikipedia is one of the trail wikis. Go to this [1] link and search for "Use Legacy Vector". Uncheck this and you will start seeing the new look on Wikipedia.

NOTE: It looks like you might have to do this for each MediaWiki site for now, but it is not too difficult if you are intentionally trying to opt-in.

[1] : https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsec...